Fidele - since your function signature is
function [a,b,c] = trial_function(num)
then you need to assign values to each of the three output variables which you are not currently doing
The easiest solution is to just assign default values to these three variables as
a = 0;
b = 0;
c = 0;
But is the above code really what you want? Do you want to assign different values to a and b and c if num is 2 or 4 or any other integer?